It is known to apply a coating to metallic substrates which are sensitive to corrosion, in order to protect them against environmental factors.
Examples of substrates requiring protection include hulls, interior and superstructures of ships, off-shore oil drilling platforms, metallic frames, bridges, automobile bodies and related equipment, storage tanks, guardrails and aircraft equipment. All these substrates, when insufficiently protected, undergo chemical transformation actions which are initially superficial but subsequently reduce their physical and particularly their mechanical characteristics, when they are exposed to severe natural or synthetic environmental conditions, such as heat, cold, ultraviolet radiation, moisture, particularly saline, wind, rain, sea water, snow, particle impacts and other harmful factors due to atmospheric pollution or to liquid or gaseous fluids resulting from the chemical, biochemical, biological and other industries.
Zinc-rich coatings containing zinc dust have been considered as the optimum anti-corrosion coatings on iron or steel substrates. However such coatings require an extremely high ratio of zinc dust to be effective and consequently their use has been restricted.
In the marine industry advances in antifouling coating technology are driven by environmental regulations and extended service requirements.
Fouling involves the formation of microbial biofilms and when these biofilms form on the hull of a vessel this may lead to increased drag and hence increased fuel consumption.
Coating compositions containing biocides such as tributyltin (TBT) compounds have been banned by the International Marine Organization (IMO) which leaves ablative copper coatings as those that are used primarily throughout the industry.
However, virtually all cuprous oxide coatings contain large amounts of copper to ensure the inhibition of fouling growth. These coatings typically include a rosin which causes the coating to leach such that a fresh surface of copper is exposed. This process continues until all of the copper has been leached after which a new coating must be applied. The ablated copper pollutes the world's ports, harms fish and impacts on the quality of the food chain.
Zinc flake, has been suggested for use in a number of anti-corrosion compositions. For example U.S. Pat. No. 5,338,348 discloses a coating composition for use in protecting metallic substrates from corrosion, comprising in weight percent, based on the total weight of the composition: from about 7% to 35% of film-forming substance for example a silicate, epoxy resin, vinyl chloride resin, polyurethane resin, acrylic or methacrylic ester polymer or an emulsion polymer; from about 35% to 55% of zinc powder; from about 5% to 25% of zinc flakes; from about 1% to 5% of at least one kind of amorphous silica; and up to about 30% particulate ferrophosphate whilst U.S. Pat. No. 5,334,631 describes a resin-based coating composition containing a mixture of zinc powder and zinc flake, epoxy resins being the preferred resins but polyesters, polyacrylates and polyurethanes being mentioned as possible alternatives.
The object of the present invention is to develop an effective environmentally safe improved coating composition and this has been achieved by providing a composition rich in zinc flake.